link to faq entry in go.mod

Author: evanw

go.mod

@@ -4,4 +4,7 @@ module github.com/evanw/esbuild
 // themselves for old OS versions. Please do not change this.
 go 1.13
 
+// This dependency cannot be upgraded or esbuild would no longer
+// compile with Go 1.13. Please do not change this. For more info,
+// please read this: https://esbuild.github.io/faq/#old-go-version
 require golang.org/x/sys v0.0.0-20220715151400-c0bba94af5f8